This invention relates to the control of the sodium carbonate concentration of the green liquor of the Kraft recovery process.
In the production of paper pulp, cellulosic fibrous material, generally wood chips, is digested in an NaOH-containing solution (white liquor). This digestion yields pulp and, as a by-product, black liquor. The Kraft recovery process is widely used to recover, from the black liquor, chemicals used in the production of paper pulp and recycle these chemicals to the pulping process. Briefly, black liquor is concentrated and then combusted to produce smelt. In a dissolving tank, the smelt is dissolved in weak wash to form green liquor, the primary component of which is Na.sub.2 CO.sub.3. The green liquor is then reacted with lime (CaO) in a causticizing reaction to convert Na.sub.2 CO.sub.3 to NaOH. The reacted mixture then passes to a clarifier which separates the liquid phase, which is essentially the white liquor, from the solid phase which is primarily CaCO.sub.3 and is referred to as lime mud. The white liquor is recycled and used for digestion of wood chips at the beginning of the cycle. The lime mud is washed to remove traces of white liquor prior to being passed to a lime kiln where CaO is regenerated. The overflow water from the washing of the lime mud is the weak wash used to dissolve the smelt in the dissolving tank.
It is desirable to have a highly efficient causticizing reaction as this increases the yield of white liquor and brings about a number of benefits in energy efficiency, productivity, and reduced emission of air pollutants. In a highly efficient causticizing reaction, Na.sub.2 CO.sub.3 is efficiently converted to NaOH. Ideally the causticizing reaction is regulated so that the causticity (i.e., 100.times.[NaOH]/([NaOH]+[Na.sub.2 CO.sub.3 ]), wherein all concentrations are expressed as Na.sub.2 O) is maintained near its equilibrium value. For most Kraft mills the equilibrium value is between 80% and 90% causticity. The efficiency of causticization can be increased in a number of ways including increasing the length of the causticization reaction, and balancing the CaO feed with the sodium carbonate feed (from the green liquor in the dissolving tank).
Attempts have been made to improve the causticizing efficiency by measuring the sodium carbonate concentration of both the green and white liquors and using this information to adjust the rate of CaO addition so as to maintain a particular level of causticization in the white liquor.
Alternatively, the sodium carbonate concentration of the green liquor is controlled. In adjusting the sodium carbonate concentration of the green liquor, the weak wash flow into the dissolving tank is the controlled variable.
Thus, if the sodium carbonate concentration in the dissolving tank is too high, the weak wash flow is increased; and when the sodium carbonate concentration is too low, the weak wash flow is decreased. Conventionally, the density of the green liquor, not the Na.sub.2 CO.sub.3 concentration, is used to determine the need for increased or decreased dilution of the green liquor.
Speaks et al. (U.S. application Ser. No. 84/00739) describe a process and apparatus for measuring the carbonate concentration in the green liquor, the white liquor, the slaker, and the causticizer and then using this information to control the causticizing reaction. As a part of this process, the concentration of Na.sub.2 CO.sub.3 in the green liquor is measured and regulated. The Na.sub.2 CO.sub.3 concentration in the green liquor is determined by drawing off and filtering green liquor, collecting a sample of filtered green liquor, reacting the sample with acid thereby producing carbon dioxide and hydrogen sulfide, and then measuring the carbon dioxide so produced using a gas chromatograph. The process takes 15-20 minutes and it is suggested the results of this analysis may be used as a set point control input to primary controllers such as conductivity probes or density gauges.
Hultmann et al. (U.S. Pat. No. 4,311,666) describe an apparatus for controlling causticization by determining the sodium carbonate concentration of both the green liquor and the white liquor, and using this information to adjust the CaO flow to the slaker. In this approach, the sodium carbonate concentration of the green liquor is determined by acidifying a sample of green liquor and measuring the carbon dioxide liberated.